Author(s): Kunihiko Hamagami, Satsuki Yamaki, Masayoshi Harada, Akinori Ozaki, Toshinori Tabata, Kazuaki Hiramatsu
Keywords: Reservoir; Blue-green algae; Water temperature stratification; Movement speed; Light intensity;
Abstract: In irrigation reservoirs, especially in the summer season, circulation is suppressed along with the stratification of the water temperature, and water quality problems such as poor oxygenation in the lower layer are likely to occur. Representative water quality problems in reservoirs create algae bloom. Algae bloom occurs frequently in the summer, causing landscape deterioration, offensive odor problems, and fish mortality. Since most algae bloom is due to abnormal proliferation of blue-green algae, it is important to grasp the characteristics of blue-green algae in order to control development. In this study, short-term behavior of blue-green algae in a shallow water body, like an agricultural reservoir, was studied. Focusing on hydraulic factors in particular, we measured the water temperature stratification during the daytime and mixing during the nighttime in the reservoir. In addition, we conducted a comparative study on the difference in the diurnal behavior between blue-green algae and other algae. As the result of the field observation on the behavior of blue-green algae in the reservoir with strong water temperature stratification, most of the algae were concentrated close to 1.3 m to 1.9 m under the mixed layer, and from that part only a group of blue-green algae diverged between this layer and the water surface. The floating speed was negatively correlated with buoyancy frequency, showing the strength of the water temperature stratification. In addition, the floating speed of the blue-green algae became larger than the past laboratory experiment results. Since vertical mixed flow works as an external force in the field, it is considered that the floating speed increases more than the buoyancy ability of the blue-green algae. In addition, although the sinking rate of the blue-green algae has a low correlation with the buoyancy frequency as compared with the floating speed, it has a certain relation with the light intensity in the water and it seems that it depends on necessary light intensity for synthesis of ballast.